The present invention relates to an extrusion machine for making hollow elongated articles of concrete and more specifically to a concrete extruder apparatus that provides improved compacting of the concrete without undue noise.
The extruding of concrete by means of one or more extruder screws into an elongated article, sometimes referred to as a slab or plank, which has hollow cavities therein to provide the minimum amount of concrete to provide the required strength, is known. One example of such a forming machine is disclosed in U.S. Pat. No. 4,548,565. In this machine, concrete is forced through extruder screws onto a deck. The machine has wheels that ride on tracks on each side of the deck. As the concrete passes from the extruder screws, the machine moves along the tracks, the movement caused by the extruding concrete. A non-rotatable forming element or die former is provided to form the hollow cavities immediately following the downstream end of the extruder screws. Thus the concrete slab is formed and slowly hardens on the deck as the machine moves along the tracks. Side plates and a top plate are provided to control the external dimensions of the slab and the apparatus moves slowly so that the concrete does not collapse. In this way, concrete slabs of almost any length can be produced. These slabs are later cut by means of a diamond saw into required lengths.
After extrusion and before the concrete hardens, it is necessary to compact the concrete and remove any air trapped therein. This is generally achieved by vibration. The vibration helps to remove air in the wet concrete which rises to the top surface of the slab. It is known to provide a vibrator on the top plate of the forming machine. This top plate controls the depth of the slab after it has been extruded. A vibrator is also provided in or adjacent the die former to compact the concrete and release air from the hollows and voids in the concrete. In most of the existing concrete extrusion machines it has been found that high frequency vibration is required. In order to achieve this a frequency converter is positioned on the machine to provide high frequency for motors to rotate at high speeds to drive vibrators on the top plate and in the die former. In most cases the motors have rotation speeds in the order of 12,000 rpm. Whereas this arrangement works well, the converters produce an excessive noise causing an unpleasant environment for operators and others. Attempts to provide lower vibration frequencies on the top plate of the extrusion machine have been partially successful, but lower vibration frequencies on the vibrator in the die former has not provided satisfactory compacting of the concrete. The die former used in existing machines is a rigid steel unit, high frequencies are transmitted through the die former to the uncured, zero slump concrete, but low frequency does not provide sufficient vibration to fully compact the formed slab.
A vibration system presently available for die formers includes an eccentric weight on a shaft that rotates inside the die former at speeds as high as 12,000 rpm. The die former is made from steel and is rigid, thus high frequency vibrations are required in order to transmit the vibration through the die former to the concrete to provide compaction and air release.